Infrared Refractive Index of Thin YBa₂Cu₃0₇ Superconducting Films

This work investigates whether thin-film optics with a constant refractive index can be applied to high-T_c superconducting thin films. The reflectance and transmittance of YBa₂Cu₃O₇ films on LaAlO₃ substrates are measured using a Fourier-transform infrared spectrometer at wavelengths from 1 to 100 μm at room temperature. The reflectance of these superconducting films at 10 K in the wavelength region from 2.5 to 25 μm is measured using a cryogenic reflectance accessory. The film thickness varies from 10 to 200 nm. By modeling the frequency-dependent complex conductivity in the normal and superconducting states and applying electromagnetic-wave theory, the complex refractive index of YBa₂Cu₃O₇ films is obtained with a fitting technique. It is found that a thickness-independent refractive index can be applied even to a 25 nm film, and average values of the spectral refractive index for film thicknesses between 25 and 200 nm are recommended for engineering applications.